Voltage detection apparatus

ABSTRACT

A voltage detection apparatus for an assembled battery includes: a capacitor; input-side switches provided between the capacitor and detection targets each including at least one battery cell; a voltage detection unit configured to detect the voltage of the capacitor; output-side switches provided between the capacitor and the voltage detection unit; a capacitor charging unit configured to charge the capacitor using a specific detection target; a first-voltage acquiring unit configured to acquire a first voltage which is the voltage of the capacitor after the passage of a first period from the charging of the capacitor; a second-voltage acquiring unit configured to acquire a second voltage which is the voltage of the capacitor after the passage of a second period from the acquisition of the first voltage; and a fault determining unit configured to determine, based on the acquired first and second voltages, whether a leak fault has occurred in the capacitor.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority from Japanese PatentApplication No. 2019-032142 filed on Feb. 25, 2019, the contents ofwhich are hereby incorporated by reference in their entirety into thisapplication.

BACKGROUND 1 Technical Field

The present disclosure relates to a voltage detection apparatus for anassembled battery that has a plurality of battery cells electricallyconnected in series with each other.

2 Description of Related Art

There is known a double flying-capacitor type voltage detectionapparatus that detects, using two capacitors, the voltage of each ofdetection targets into which a plurality of battery cells are divided.More specifically, each of the detection targets includes at least oneof the battery cells. The two capacitors are used to acquire thevoltages of different ones of the detection targets at the same time.Moreover, the voltage detection apparatus further determines, based onthe acquired voltages of the detection targets, whether a leak fault hasoccurred in either of the two capacitors.

SUMMARY

According to the present disclosure, there is provided a voltagedetection apparatus for an assembled battery. The assembled battery hasa plurality of battery cells electrically connected in series with eachother. The battery cells are divided into a plurality of detectiontargets. Each of the detection targets includes at least one of thebattery cells. The voltage detection apparatus includes a capacitor,input-side switches, a voltage detection unit, output-side switches, acapacitor charging unit, a first-voltage acquiring unit, asecond-voltage acquiring unit and a fault determining unit. Theinput-side switches are provided to parallel connect the detectiontargets to the capacitor and open and close electrical paths between thedetection targets and the capacitor. The voltage detection unit isconfigured to detect the voltage of the capacitor. The output-sideswitches are provided to open and close electrical paths between thecapacitor and the voltage detection unit. The capacitor charging unit isconfigured to turn the output-side switches to an open state and thoseof the input-side switches which correspond to a specific detectiontarget to a closed state and thereby charge the capacitor with electricpower stored in the specific detection target. The specific detectiontarget is one of the plurality of detection targets. The first-voltageacquiring unit is configured to turn, after the charging of thecapacitor by the capacitor charging unit, the input-side switchescorresponding to the specific detection target to an open state and theoutput-side switches to a closed state and thereby acquire a firstvoltage. The first voltage is the voltage of the capacitor after a firstperiod has elapsed from the turning of the input-side switchescorresponding to the specific detection target to the open state. Thesecond-voltage acquiring unit is configured to acquire a second voltage.The second voltage is the voltage of the capacitor after a second periodhas elapsed from the acquisition of the first voltage by thefirst-voltage acquiring unit. The fault determining unit is configuredto determine, based on the first voltage acquired by the first-voltageacquiring unit and the second voltage acquired by the second-voltageacquiring unit, whether a leak fault has occurred in the capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating the overall configuration ofan electric power supply system which includes a voltage detectionapparatus according to a first embodiment;

FIG. 2 is a flow chart illustrating a voltage detection processperformed by a control unit of the voltage detection apparatus;

FIG. 3 is a graphical representation illustrating the relationshipbetween a second period, a first voltage and a threshold value;

FIG. 4 is a schematic diagram illustrating the time schedule of thevoltage detection process;

FIG. 5 is a time chart illustrating (a) change in the processesperformed by the control unit, (b) change in the open/closed state oftarget switches, (c) change in the open/closed state of output-sideswitches and (d) change in the voltage of a capacitor of the voltagedetection apparatus, all the changes being caused by execution of thevoltage detection process by the control unit;

FIG. 6 is a time chart illustrating the advancing of first-voltageacquisition timings in the voltage detection process when a leak faulthas occurred in the capacitor;

FIG. 7 is a graphical representation illustrating the relationshipbetween the leak degree of the capacitor and the amount of leakdischarge from the capacitor per unit time;

FIG. 8 is a schematic diagram illustrating the overall configuration ofan electric power supply system which includes a voltage detectionapparatus according to a second embodiment;

FIG. 9 is a schematic diagram illustrating the time schedule of avoltage detection process performed by a control unit of the voltagedetection apparatus according to the second embodiment; and

FIG. 10 is a time chart illustrating a voltage detection processaccording to a modification.

DESCRIPTION OF EMBODIMENTS

As flying-capacitor type voltage detection apparatuses, there is alsoknown a single flying-capacitor type voltage detection apparatus inaddition to the above-described double flying-capacitor type voltagedetection apparatus known in the art (see, for example, Japanese PatentApplication Publication No. JP2017156297A). Unlike the doubleflying-capacitor type voltage detection apparatus, the singleflying-capacitor type voltage detection apparatus includes only a singlecapacitor. Therefore, it is impossible for the single flying-capacitortype voltage detection apparatus to acquire the voltages of twodifferent detection targets at the same time. Consequently, it isimpossible for the single flying-capacitor type voltage detectionapparatus to detect a leak fault of the single capacitor using the samemethod as described in Japanese Patent Application Publication No.JP2017156297A.

In contrast, in the above-described voltage detection apparatusaccording to the present disclosure, the voltage of the capacitor, whichhas been charged with electric power stored in the specific detectiontarget, is acquired twice at different timings respectively as the firstvoltage and the second voltage. Then, based on the acquired first andsecond voltages, it is determined whether a leak fault has occurred inthe capacitor. That is, the leak fault determination is made for thecapacitor using the capacitor itself. Consequently, it becomes possibleto suitably make the leak fault determination for the capacitor even ifthe voltage detection apparatus includes only the single capacitor.

In further implementations, the voltage detection apparatus may furtherinclude a switch-state control unit that is configured to turn theoutput-side switches to the open state during the second period from theacquisition of the first voltage by the first-voltage acquiring unit tothe acquisition of the second voltage by the second-voltage acquiringunit.

With the above configuration, it is possible to suppress discharge fromthe capacitor to the voltage detection unit side during the secondperiod. That is, any discharge from the capacitor other than leakdischarge can be suppressed. Consequently, it becomes possible toaccurately determine whether a leak fault has occurred in the capacitor.

In the voltage detection apparatus, the fault determining unit may beconfigured to determine, when a voltage difference between the firstvoltage acquired by the first-voltage acquiring unit and the secondvoltage acquired by the second-voltage acquiring unit is larger than apreset threshold value, that a leak fault has occurred in the capacitor.The threshold value may be preset based on at least one of the firstvoltage acquired by the first-voltage acquiring unit and the secondperiod.

When a leak fault has occurred in the capacitor, the higher the firstvoltage and the longer the second period, the larger the amount of leakdischarge from the capacitor per unit time and thus the larger thevoltage difference between the first and second voltages. Therefore,presetting the threshold value, which is used in the leak faultdetermination for the capacitor, based on at least one of the firstvoltage and the second period, it is possible to more accurately makethe leak fault determination for the capacitor than in the case ofpresetting the threshold value to a constant value regardless of thefirst voltage and the second period Y2.

The voltage detection apparatus may further include a detection-targetvoltage acquiring unit that is configured to acquire the voltage of thespecific detection target based on the first voltage acquired by thefirst-voltage acquiring unit.

With the above configuration, the voltage of the specific detectiontarget is acquired (or determined) based on the first voltage that hasbeen acquired for making the leak fault determination for the capacitor.Therefore, it is unnecessary to further acquire, in addition to thefirst and second voltages, the voltage of the capacitor for the purposeof acquiring the voltage of the specific detection target. Consequently,it is possible to simplify control of the entire voltage detectionapparatus.

Moreover, the voltage detection apparatus may further include afirst-period setting unit that is configured to set the first period tobe shorter when it is determined by the fault determining unit that aleak fault has occurred in the capacitor than when it is determined thatno leak fault has occurred in the capacitor.

When a leak fault has occurred in the capacitor, the shorter the firstperiod, the smaller the amount of leak discharge from the capacitor perunit time and thus the less the influence of the leak fault on theacquired voltage of the specific detection target. Therefore, shorteningthe first period when it is determined that a leak fault has occurred inthe capacitor, it is possible to more accurately acquire the voltage ofthe specific detection target; thus it is also possible to moreaccurately estimate the SOC of the specific detection target on thebasis of the acquired voltage of the specific detection target.

The voltage detection apparatus may further include a second-periodsetting unit that is configured to set the second period to be longerwhen it is determined by the fault determining unit that a leak faulthas occurred in the capacitor than when it is determined that no leakfault has occurred in the capacitor.

When a leak fault has occurred in the capacitor, the longer the secondperiod, the larger the amount of leak discharge from the capacitor perunit time and thus the higher the accuracy of estimation of the leakdegree of the capacitor based on the amount of leak discharge.Therefore, lengthening the second period when it is determined that aleak fault has occurred in the capacitor, it is possible to moreaccurately estimate the leak degree of the capacitor; thus it is alsopossible to more suitably determine, based on the estimated leak degree,whether it is necessary to replace the capacitor.

Exemplary embodiments will be described hereinafter with reference tothe drawings. It should be noted that for the sake of clarity andunderstanding, identical components having identical functionsthroughout the whole description have been marked, where possible, withthe same reference numerals in each of the figures and that for the sakeof avoiding redundancy, descriptions of identical components will not berepeated.

First Embodiment

FIG. 1 shows the overall configuration of an electric power supplysystem 100 which includes a voltage detection apparatus 20 according tothe first embodiment. The electric power supply system 100 is installedin, for example, a hybrid vehicle or an electric vehicle.

As shown in FIG. 1, in the present embodiment, the electric power supplysystem 100 includes an assembled battery 10 and the voltage detectionapparatus 20 that is of a flying capacitor type.

The assembled battery 10 is an electric power source for supplyingelectric power to in-vehicle electrical loads which include, forexample, a rotating electric machine (not shown) provided for propulsionof the vehicle.

The assembled battery 10 has a plurality of battery cells electricallyconnected in series with each other. The terminal voltage of theassembled battery is, for example, several hundred volts. The batterycells may be implemented by, for example, storage batteries such aslithium-ion batteries or nickel-metal hydride batteries.

More particularly, in the present embodiment, the assembled battery 10has a plurality (e.g., eight) of battery modules B electricallyconnected in series with each other. Each of the battery modules B isformed by integrating at least two battery cells, which are electricallyconnected in series with each other, into one piece. For the sake ofconvenience of explanation, hereinafter, the eight battery modules Bconstituting the assembled battery 10 will be sequentially referred toas the first battery module B1, the second battery module B2, . . . ,and the eighth battery module B8 from the highest electric potentialside. All the battery modules B1-B8 include the same number of batterycells; thus all the rated voltages of the battery modules B1-B8 areequal to each other.

Moreover, in the present embodiment, the battery modules B1-B8 aredivided into first to sixth detection targets A1-A6. Each of thedetection targets A1-A6 includes at least one of the battery modulesB1-B8. Specifically, the first detection target A1 includes only thefirst battery module B1. The second detection target A2 includes onlythe second battery module B2. The third detection target A3 includesboth the third and fourth battery modules B3 and B4 that areelectrically connected in series with each other. The fourth detectiontarget A4 includes both the fifth and sixth battery modules B5 and B6that are electrically connected in series with each other. The fifthdetection target A5 includes only the seventh battery module B7. Thesixth detection target A6 includes only the eighth battery module B8.

In the assembled battery 10, there are provided first to seventhelectrode terminals T1-T7. The number of the electrode terminals T1-T7is greater than the number of the detection targets A1-A6 by one.Moreover, the mth detection target Am has its positive terminalconnected with the mth electrode terminal Tm and its negative terminalconnected with the (m+1)th electrode terminal T(m+1), where m is anarbitrary integer in the range of 1 to 6.

The voltage detection apparatus 20 has a resistor section 21, aninput-side switch section 22, a capacitor section 23, an output-sideswitch section 24, a voltage detection unit 25 and a control unit 27.

The resistor section 21 is provided between the assembled battery 10 andthe input-side switch section 22. The resistor section 21 includes sevencurrent-limiting resistors R each of which is individually provided on adetection line Ln connected with the nth electrode terminal Tn, where nis an arbitrary integer in the range of 1 to 7. The current-limitingresistors R are provided to prevent inrush current from flowing from theassembled battery 10 (i.e., the higher voltage side) to the input-sideswitch section 22 (i.e., the lower voltage side). The resistances of thecurrent-limiting resistors R are equal to each other.

The input-side switch section 22 is provided between the resistorsection 21 and the capacitor section 23. The input-side switch section22 includes first to seventh switches SW1-SW7 each of which is connectedwith a corresponding one of the detection lines Ln. Each of the switchesSWn of the input-side switch section 22 is provided to connect anddisconnect a corresponding one of the electrode terminals Tn to and fromthe capacitor section 23. In addition, each of the switches SWn may beimplemented by, for example, a pair of N-channel MOSFETs having theirsources connected with each other, a photo relay or an electric relay.

The capacitor section 23 includes a single capacitor CA. That is, thevoltage detection apparatus 20 according to the present embodiment is asingle flying-capacitor type voltage detection apparatus. The capacitorCA has first and second terminals N1 and N2 provided as connectionterminals. Each of the electrode terminals Tn of the assembled battery10 is connected with one of the first and second terminals N1 and N2 viaa corresponding one of the switches SWn.

Specifically, to the first terminal N1 of the capacitor CA, there areconnected the second, fourth and sixth electrode terminals T2, T4 and T6respectively via the second, fourth and sixth switches SW2, SW4 and SW6.On the other hand, to the second terminal N2 of the capacitor CA, thereare connected the first, third, fifth and seventh electrode terminalsT1, T3, T5 and T7 respectively via the first, third, fifth and seventhswitches SW1, SW3, SW5 and SW7. That is, the switches SW1-SW7 areprovided to parallel connect the detection targets A1-A6 to thecapacitor CA and open and close electrical paths between the detectiontargets A1-A6 and the capacitor CA.

The output-side switch section 24 is provided between the capacitorsection 23 and the voltage detection unit 25. The output-side switchsection 24 includes switches SWA and SWB that are respectively connectedwith the first and second terminals N1 and N2 of the capacitor CA.Specifically, the switch SWA is connected with the first terminal N1while the switch SWB is connected with the second terminal N2. Theswitches SWA and SWB are provided to open and close electrical pathsbetween the capacitor CA and the voltage detection unit 25. In addition,each of the switches SWA and SWB may be implemented by, for example, asemiconductor switch such as an N-channel MOSFET.

The voltage detection unit 25 is provided between the output-side switchsection 24 and the control unit 27. The voltage detection unit 25 isconnected in parallel with the capacitor CA via the switches SWA andSWB. The voltage detection unit 25 is provided to detect the voltage ofthe capacitor CA. Specifically, though not shown in the figures, thevoltage detection unit 25 includes a differential amplification circuitand an AD conversion circuit. The voltage detection unit 25 detects thevoltage of the capacitor CA using the differential amplificationcircuit, and outputs the detected voltage to the control unit 27 throughan AD conversion by the AD conversion circuit.

The control unit 27 is configured with a microcomputer which includes aCPU and a memory. The control unit 27 is provided to control the on/offof each of the switches SWn, the on/off of each of the switches SWA andSWB and voltage acquisition timings TD1 and TD2 (see FIG. 5) of thevoltage detection unit 25. Specifically, the control unit 27 performs avoltage detection process which includes charge processes andacquisition processes. In each of the charge processes, with thecapacitor CA and the voltage detection unit 25 electrically insulatedfrom each other, the capacitor CA is charged using one of the detectiontargets A1-A6 of the assembled battery 10. In each of the acquisitionprocesses, with the assembled battery 10 and the capacitor CAelectrically insulated from each other, the voltage of one of thedetection targets A1-A6, with which the capacitor CA has been charged inthe charge process immediately before the acquisition process, isacquired based on the detected voltage outputted from the voltagedetection unit 25.

FIG. 2 illustrates the voltage detection process according to thepresent embodiment. This process is repeatedly performed by the controlunit 27 in a predetermined cycle. In addition, at the start of thevoltage detection process, all of the switches SWn and the switches SWAand SWB are in an open state (i.e., off-state).

In the voltage detection process, first, in step S10, it is determinedwhether one of predetermined processing timings TS has been reached.Each of the processing timings TS is predetermined, for a correspondingone of the detection targets A1-A6, as a timing for performing a firstprocess or a second process for the corresponding detection target.

If the determination in step S10 results in a “NO” answer, the processdirectly terminates without performing the remaining steps.

In contrast, if the termination in step S10 results in a “YES” answer,the process proceeds to step S12.

In step S12, it is determined whether the current voltage detectiontarget is the sixth detection target A6.

In addition, in the present embodiment, the sixth detection target A6corresponds to a “specific detection target”.

If the determination in step S12 results in a “NO” answer, i.e., if thecurrent voltage detection target is one of the detection targets A1-A5,the process proceeds to step S12 to perform the first process for thecurrent voltage detection target.

The first process includes the charge process for charging the capacitorCA with electric power stored in the current voltage detection target(i.e., one of the detection targets A1-A5), and the acquisition processfor acquiring the voltage of the current voltage detection target by thevoltage detection unit 25.

Hereinafter, for the sake of avoiding redundancy, explanation will bemade only for the case of the current voltage detection target being,for example, the first detection target A1.

In step S14, the charge process is performed. Specifically, in thecharge process, both the first and second switches SW1 and SW2, whichare respectively connected with the first and second electrode terminalsT1 and T2, are turned to a closed state (or on-state), thereby chargingthe capacitor CA with electric power stored in the first detectiontarget A1. In addition, the first and second electrode terminals T1 andT2 are respectively connected with the positive and negative terminalsof the first detection target A1.

After a predetermined charge period has elapsed, in step S16, both thefirst and second switches SW1 and SW2 (denoted by SWn in FIG. 2) areturned to the open state.

Upon turning both the first and second switches SW1 and SW2 to the openstate, the acquisition process is performed. Specifically, in theacquisition process, before a predetermined first period Y1 has elapsedfrom the turning of both the first and second switches SW1 and SW2 tothe open state, in step S18, both the switches SWA and SWB are turned toa closed state (or on-state), thereby allowing the voltage of thecapacitor CA to be acquired by the voltage detection unit 25.

In step S20, at a first-voltage acquisition timing TD1 after the firstperiod Y1 has elapsed from the turning of both the first and secondswitches SW1 and SW2 to the open state, the voltage of the capacitor CAis acquired as a first voltage V1 by the voltage detection unit 25 (seeFIG. 5).

After the acquisition of the first voltage V1, in step S22, both theswitches SWA and SWB are turned to the open state.

In step S24, the voltage of the first detection target A1 is acquiredbased on the first voltage V1 acquired in step S20. Then, the voltagedetection process terminates.

On the other hand, if the determination in step S12 results in a “YES”answer, i.e., if the current voltage detection target is the sixthdetection target A6, the process proceeds to step S26 to perform thesecond process.

The second process includes the charge process for charging thecapacitor CA with electric power stored in the sixth detection targetA6, the acquisition process for acquiring the voltage of the sixthdetection target A6 by the voltage detection unit 25, and adetermination process for determining whether a leak fault has occurredin the capacitor CA.

In step S26, the charge process is performed. Specifically, in thecharge process, both the sixth and seventh switches SW6 and SW7, whichare respectively connected with the sixth and seventh electrodeterminals T6 and T7, are turned to a closed state (or on-state), therebycharging the capacitor CA with electric power stored in the sixthdetection target A6. In addition, the sixth and seventh electrodeterminals T6 and T7 are respectively connected with the positive andnegative terminals of the sixth detection target A6.

After a predetermined charge period has elapsed, in step S28, both thesixth and seventh switches SW6 and SW7 (denoted by SWn in FIG. 2) areturned to the open state.

Upon turning both the sixth and seventh switches SW6 and SW7 to the openstate, the acquisition process is performed. Specifically, in theacquisition process, before a predetermined first period Y1 has elapsedfrom the turning of both the sixth and seventh switches SW6 and SW7 tothe open state, in step S30, both the switches SWA and SWB are turned tothe closed state, thereby allowing the voltage of the capacitor CA to beacquired by the voltage detection unit 25.

In step S32, at a first-voltage acquisition timing TD1 after the firstperiod Y1 has elapsed from the turning of both the sixth and seventhswitches SW6 and SW7 to the open state, the voltage of the capacitor CAis acquired as the first voltage V1 by the voltage detection unit 25(see FIG. 5).

After the acquisition of the first voltage V1, in step S34, both theswitches SWA and SWB are turned to the open state.

In step S36, the voltage of the sixth detection target A6 is acquiredbased on the first voltage V1 acquired in step S32. Then, the processproceeds to step S38.

In addition, in the present embodiment, the control unit 27 functions asa “capacitor charging unit” to perform above step S26, as a“first-voltage acquiring unit” to perform above step S32, and as a “adetection-target voltage acquiring unit” to perform above step S36.

After the acquisition of the voltage of the sixth detection target A6 instep S36, the determination process is performed to determine whether aleak fault has occurred in the capacitor CA.

Specifically, in step S38, before a predetermined second period Y2 haselapsed from the acquisition of the first voltage V1 in step S32, boththe switches SWA and SWB are turned to the closed state.

Then, in step S40, at a second-voltage acquisition timing TD2 after thesecond period Y2 has elapsed from the acquisition of the first voltageV1 in step S32, the voltage of the capacitor CA is acquired as a secondvoltage V2 by the voltage detection unit 25 (see FIG. 5).

After the acquisition of the second voltage V2, in step S42, both theswitches SWA and SWB are turned to the open state.

In addition, in the present embodiment, the control unit 27 functions asa “switch-state control unit” to perform above steps S34 and S38, and asa “second-voltage acquiring unit” to perform above step S40.

In step S44, based on both the first voltage V1 acquired in step S32 andthe second voltage V2 acquired in step S40, it is determined whether aleak fault has occurred in the capacitor CA.

Specifically, in this step, a voltage difference ΔV between the firstvoltage V1 and the second voltage V2 is calculated by the followingEquation 1. Then, it is determined whether the voltage difference ΔV islarger than a preset threshold value Vth.

ΔV=|V1−V2|  (Equation 1)

In addition, in the present embodiment, the control unit 27 functions as“fault determining unit” to perform above step S44.

The threshold value Vth is preset to an amount of decrease in thevoltage of the capacitor CA which may be caused, when no leak fault hasoccurred in the capacitor CA, by natural discharge of the capacitor CAduring the second period Y2. More particularly, the threshold value Vthis preset to the maximum amount of decrease in the voltage of thecapacitor CA which may be caused by natural discharge of the capacitorCA during the second period Y2.

In the present embodiment, the threshold value Vth is preset based onboth the first voltage V1 acquired in step S32 and the second period Y2.FIG. 3 illustrates the relationship between the second period Y2, thefirst voltage V1 and the threshold value Vth. As shown in FIG. 3, thelonger the second period Y2, the larger the threshold value Vth ispreset to be; the higher the first voltage V1, the larger the thresholdvalue Vth is preset to be.

When the voltage difference ΔV is not larger than the threshold valueVth, it is determined that no leak fault has occurred in the capacitorCA. That is, the determination in step S44 results in a “NO” answer. Inthis case, the process directly terminates without performing theremaining steps.

In contrast, when the voltage difference ΔV is larger than the thresholdvalue Vth, it is determined that a leak fault has occurred in thecapacitor CA. That is, the determination in step S44 results in a “YES”answer. In this case, the process proceeds to step S46.

In step S46, the first period Y1 is shortened in comparison with thecase where it is determined that no leak fault has occurred in thecapacitor CA. That is, the time period from the turning of both thesixth and seventh switches SW6 and SW7 to the open state to theacquisition of the first voltage V1 is shortened.

In addition, in the present embodiment, the control unit 27 functions asa “first-period setting unit” to perform above step S46.

In subsequent step S48, the second period Y2 is lengthened in comparisonwith the case where it is determined that no leak fault has occurred inthe capacitor CA. That is, the time period from the acquisition of thefirst voltage V1 to the acquisition of the second voltage V2 islengthened. Then, the voltage detection process terminates.

In addition, in the present embodiment, the control unit 27 functions asa “second-period setting unit” to perform above step S48.

Moreover, in the case where steps S46 and S48 have been performed in theprevious execution of the voltage detection process, instead ofperforming steps S46 and S48 in the present execution of the voltagedetection process, a determination is made, based on the voltagedifference ΔV, as to whether it is necessary to replace the capacitorCA.

FIG. 4 illustrates the time schedule of the voltage detection processaccording to the present embodiment.

As shown in FIG. 4, in the voltage detection process according to thepresent embodiment, the voltages of the first to the sixth detectiontargets A1-A6 are sequentially acquired in this order and then thevoltage detection unit 25 is calibrated. Specifically, in thecalibration of the voltage detection unit 25, the offset of a referencevoltage inputted to the differential amplification circuit included inthe voltage detection unit 25 is calibrated. Therefore, the processingperiod YR for performing the voltage detection process includes sixdetection periods YD and one calibration period YP. The processingperiod YR is, for example, several tens of milliseconds.

The start timing of each of the detection periods YD is the processingtiming TS. For each of the first to the fifth detection targets A1-A5,upon reaching the processing timing TS, the first process is performedwithin the corresponding detection period YD. On the other hand, for thesixth detection target A6, upon reaching the processing timing TS, thesecond process is performed within the corresponding detection period YDand the subsequent calibration period YP.

In the present embodiment, the second process is performed utilizing thecalibration period YP. Consequently, it becomes possible to suppress theprocessing period YR from being extended due to the execution of thesecond process.

Of the time schedule of the voltage detection process shown in FIG. 4,the first process performed for the first detection target A1 and thesecond process performed for the sixth detection target A6 are shown inFIG. 5.

FIG. 5 is a time chart illustrating: (a) change in the processesperformed by the control unit 27; (b) change in the open/closed state oftarget switches SWn (i.e., those of the first to the seventh switchesSW1-SW7 which are closed and opened in the corresponding chargeprocesses); (c) change in the open/closed state of the switches SWA andSWB; and (d) change in the voltage of the capacitor CA.

It should be noted that the voltage of the capacitor CA shown in FIG.5(d) is represented by the relative electric potential of the firstterminal N1 to the second terminal N2 of the capacitor CA. Therefore,the voltage of the capacitor CA shown in FIG. 5(d) takes a positivevalue when the electric potential of the first terminal N1 is higherthan the electric potential of the second terminal N2, and a negativevalue when the electric potential of the first terminal N1 is lower thanthe electric potential of the second terminal N2.

In addition, in FIG. 5(d), change in the voltage of the capacitor CAwhen no leak fault has occurred in the capacitor CA is shown with asolid line whereas change in the voltage of the capacitor CA when a leakfault has occurred in the capacitor CA is shown with a dashed line.

As shown in FIG. 5(a), upon start of the first process for the firstdetection target A1 at a timing t1, the charge process is firstperformed using the first detection target A1 during the charge periodYC.

Specifically, as shown in FIG. 5(b), the target switches SWn for thefirst detection target A1 (i.e., the first and second switches SW1 andSW2) are turned to the closed state. Consequently, as shown in FIG.5(d), the capacitor CA is charged by the first detection target A1 sothat the voltage of the capacitor CA takes a negative valuecorresponding to the voltage of the first detection target A1.

Upon the passage of the charge period YC, the target switches SWn areturned to the open state at a timing t2. Then, in the subsequentacquisition period YE, the acquisition process is performed using thevoltage detection unit 25.

Specifically, as shown in FIG. 5(c), upon the passage of a predeterminedfirst waiting period YW1 from the timing t2, both the switches SWA andSWB are turned to the closed state.

In addition, by providing the first waiting period YW1 in the voltagedetection process, it becomes possible to prevent the occurrence of asituation where all of the target switches SWn and the switches SWA andSWB are in the closed state; consequently, it becomes possible toprevent overcurrent from flowing to the voltage detection unit 25.

At a timing t3 which is the first-voltage acquisition timing TD1 afterthe passage of the first period Y1 (Y1>YW1) from the turning of thetarget switches SWn (i.e., the first and second switches SW1 and SW2) tothe open state, the voltage of the capacitor CA is acquired as the firstvoltage V1.

After the acquisition of the first voltage V1 at the timing t3, both theswitches SWA and SWB are turned to the open state. Thereafter, upon thepassage of the acquisition period YE, at a timing t4, the first processfor the first detection target A1 and thus the detection period YDcorresponding to the first detection target A1 terminate. That is, thecharge period YC plus the acquisition period YE constitutes thedetection period YD.

Moreover, upon start of the second process for the sixth detectiontarget A6 at a timing t11, the charge process is first performed usingthe sixth detection target A6 during the charge period YC.

Specifically, as shown in FIG. 5(b), the target switches SWn for thesixth detection target A6 (i.e., the sixth and seventh switches SW6 andSW7) are turned to the closed state. Consequently, as shown in FIG.5(d), the capacitor CA is charged by the sixth detection target A6 sothat the voltage of the capacitor CA takes a positive valuecorresponding to the voltage of the sixth detection target A6.

Upon the passage of the charge period YC, the target switches SWn areturned to the open state at a timing t12. Then, in the subsequentacquisition period YE, the acquisition process is performed using thevoltage detection unit 25.

Specifically, as shown in FIG. 5(c), upon the passage of a predeterminedfirst waiting period YW1 from the timing t12, both the switches SWA andSWB are turned to the closed state.

Then, at a timing t13 which is the first-voltage acquisition timing TD1after the passage of the first period Y1 (Y1>YW1) from the turning ofthe target switches SWn (i.e., the sixth and seventh switches SW6 andSW7) to the open state, the voltage of the capacitor CA is acquired asthe first voltage V1.

After the acquisition of the first voltage V1 at the timing t13, boththe switches SWA and SWB are turned to the open state. Thereafter, uponthe passage of the acquisition period YE, at a timing t14, the detectionperiod YD corresponding to the sixth detection target A6 terminates.That is, the charge period YC plus the acquisition period YE constitutesthe detection period YD.

Upon the termination of the detection period YD corresponding to thesixth detection target A6, the determination process is performed duringthe calibration period YP.

Specifically, upon the passage of a predetermined second waiting periodYW2 from the timing t14, both the switches SWA and SWB are again turnedto the closed state.

Then, at a timing t15 which is the second-voltage acquisition timing TD2after the passage of the second period Y2 from the acquisition of thefirst voltage V1 at the timing t13, the voltage of the capacitor CA isacquired as the second voltage V2.

After the acquisition of the second voltage V2 at the timing t15, boththe switches SWA and SWB are turned to the open state. Thereafter, uponthe passage of the calibration period YP, at a timing t16, the secondprocess for the sixth detection target A6 and thus the entire voltagedetection process terminate.

As shown in FIG. 5(b), in the voltage detection process according to thepresent embodiment, during the acquisition periods YE and thecalibration period YP, the target switches SWn are kept in the openstate. Consequently, it becomes possible to prevent electric charge,which has been charged into the capacitor CA during the charge periodsYC, from being discharged to the assembled battery 10 side during theacquisition periods YE and the calibration period YP.

Moreover, as shown in FIG. 5(c), in the voltage detection processaccording to the present embodiment, during the acquisition periods YEand the calibration period YP, the on-periods, for which both theswitches SWA and SWB are kept in the closed state, are limited, morespecifically limited to predetermined periods respectively around thevoltage acquisition timings TD1 and TD2. Consequently, it becomespossible to suppress electric charge, which has been charged into thecapacitor CA during the charge periods YC, from being discharged to thevoltage detection unit 25 side during the acquisition periods YE and thecalibration period YP.

In particular, in the second process performed for the sixth detectiontarget A6, during the second period Y2 from the acquisition of the firstvoltage V1 to the acquisition of the second voltage V2, both theswitches SWA and SWB are turned to the open state. Consequently, asshown with a solid line in FIG. 5(d), when no leak fault has occurred inthe capacitor CA, the capacitor CA is in a high-impedance state duringthe second period Y2. As a result, during the second period Y2, thevoltage of the capacitor CA is kept approximately constant and thus thevoltage difference ΔV between the first voltage V1 and the secondvoltage V2 becomes smaller than the threshold value Vth.

In contrast, as shown with a dashed line in FIG. 5(d), when a leak faulthas occurred in the capacitor CA, the voltage of the capacitor CAconsiderably decreases due to the leak discharge from the capacitor CAand thus the voltage difference ΔV becomes larger than the thresholdvalue Vth. Therefore, it is possible to determine, through comparisonbetween the voltage difference ΔV and the threshold value Vth, whether aleak fault has occurred in the capacitor CA.

When a leak fault has occurred in the capacitor CA, the first voltagesV1 acquired at the first-voltage acquisition timings TD1 are lowered andthus the acquired voltages of the detection targets A1-A6 become lowerthan the actual voltages of the same. Moreover, when the acquiredvoltages of the detection targets A1-A6 become lower than the actualvoltages of the same, the SOCs (States of Charge) of the detectiontargets A1-A6 will be estimated to be lower than the actual valuesthereof, making it impossible to use up the electric power stored in thedetection targets A1-A6. In view of the above, in the voltage detectionprocess according to the present embodiment, when a leak fault hasoccurred in the capacitor CA, the first-voltage acquisition timings TD1for acquiring the first voltages V1 are advanced.

FIG. 6 is a time chart illustrating the advancing of the first-voltageacquisition timings TD1 when a leak fault has occurred in the capacitorCA. In addition, FIG. 6(a)-(d) are respectively similar to theabove-described FIG. 5(a)-(d) and thus will not be described in detailhereinafter.

As shown in FIG. 6(c), in the first process for the first detectiontarget A1, the first-voltage acquisition timing TD1 is advanced from thetiming t3 to a timing t23. That is, the first period Y1 is shortened byadvancing the first-voltage acquisition timing TD1.

In addition, the timing t23 is a timing after the passage of the firstwaiting period YW1 from the timing t2. Therefore, at the timing t23,both the switches SWA and SWB have already been turned to the closedstate. In the present embodiment, when a leak fault has occurred in thecapacitor CA, the first-voltage acquisition timings TD1 for acquiringthe first voltages V1 are advanced while the timings of closing andopening the switches SWA and SWB are kept unchanged.

Similarly, in the second process for the sixth detection target A6, thefirst-voltage acquisition timing TD1 is advanced from the timing t13 toa timing t33. That is, the first period Y1 is shortened by advancing thefirst-voltage acquisition timing TD1. In addition, the time differenceΔYA between the timing T23 and the timing t3 is equal to the timedifference ΔYB between the timing T33 and the timing t13.

Consequently, as shown in FIG. 6(d), the absolute values of the firstvoltages V1 acquired at the first-voltage acquisition timings TD1 areincreased by a difference value ΔVX in comparison with the case of thefirst voltages V1 being acquired without advancing the first-voltageacquisition timings TD1. As a result, it becomes possible to suppressdecrease in the estimation accuracy of the SOCs of the detection targetsA1-A6 during the time period from the occurrence of a leak fault of thecapacitor CA to the replacement of the capacitor CA. Accordingly, theadvancing of the first-voltage acquisition timings TD1 when a leak faulthas occurred in the capacitor CA is advantageous in terms of using upthe electric power stored in the assembled battery 10.

On the other hand, in the second process for the sixth detection targetA6, the second-voltage acquisition timing TD2 is kept unchanged at thetiming t15. That is, the second period Y2 is lengthened by advancing thefirst-voltage acquisition timing TD1 while keeping the second-voltageacquisition timing TD2 unchanged. Consequently, as shown in FIG. 6(d),the voltage difference ΔV between the first voltage V1 and the secondvoltage V2 is increased by the difference value ΔVX in comparison withthe case of the first voltage V1 being acquired without advancing thefirst-voltage acquisition timing TD1. Moreover, with increase in thevoltage difference ΔV, a leak degree RD can be more accuratelyestimated. Here, the leak degree RD denotes the amount of increase inthe voltage difference ΔV per unit time. The leak degree RD can beexpressed by the following Equation 2.

RD=ΔV/Y2  (Equation 2)

The leak degree RD represents the degree of the leak fault of thecapacitor CA. The leak degree RD is proportional to the amount of leakdischarge QR from the capacitor CA per unit time.

FIG. 7 illustrates the relationship between the leak degree RD and theamount of leak discharge QR from the capacitor CA per unit time. As seenfrom FIG. 7, the amount of leak discharge QR increases with the leakdegree RD. Therefore, by accurately estimating the leak degree RD, it ispossible to suitably determine, based on the estimated leak degree RD,whether it is necessary to replace the capacitor CA.

According to the present embodiment, it is possible to achieve thefollowing advantageous effects.

In the present embodiment, the voltage of the capacitor CA, which hasbeen charged with electric power stored in the sixth detection targetA6, is first acquired as the first voltage V1 at the first-voltageacquisition timing TD1 and then acquired as the second voltage V2 at thesecond-voltage acquisition timing TD2. Based on the acquired first andsecond voltages V1 and V2, it is determined whether a leak fault hasoccurred in the capacitor CA. That is, the leak fault determination ismade for the capacitor CA using the capacitor CA itself. Consequently,it becomes possible to suitably make the leak fault determination forthe capacitor CA even though the flying-capacitor type voltage detectionapparatus 20 includes only the single capacitor CA.

In the present embodiment, during the second period Y2, both theswitches SWA and SWB are turned to the open state, thereby suppressingdischarge from the capacitor CA to the voltage detection unit 25 side.That is, any discharge from the capacitor CA other than leak dischargeis suppressed. Consequently, it becomes possible to accurately determinewhether a leak fault has occurred in the capacitor CA.

When a leak fault has occurred in the capacitor CA, the higher the firstvoltage V1 and the longer the second period Y2, the larger the amount ofleak discharge QR from the capacitor CA per unit time and thus thelarger the voltage difference ΔV between the first and second voltagesV1 and V2. In view of the above, in the present embodiment, thethreshold value Vth, which is used in the leak fault determination forthe capacitor CA, is preset based on both the first voltage V1 and thesecond period Y2. Consequently, it becomes possible to more accuratelymake the leak fault determination for the capacitor CA than in the caseof presetting the threshold value Vth to a constant value regardless ofthe first voltage V1 and the second period Y2.

In the present embodiment, in the second process for the sixth detectiontarget A6, the voltage of the sixth detection target A6 is acquired (ordetermined) based on the first voltage V1 that has been acquired formaking the leak fault determination for the capacitor CA. Therefore, itis unnecessary to further acquire, in addition to the first and secondvoltages V1 and V2, the voltage of the capacitor CA for the purpose ofacquiring the voltage of the sixth detection target A6. Consequently, itbecomes possible to simplify control of the entire voltage detectionapparatus 20.

When a leak fault has occurred in the capacitor CA, in the secondprocess for the sixth detection target A6, the shorter the first periodY1, the smaller the amount of leak discharge QR from the capacitor CAper unit time and thus the less the influence of the leak fault on theacquired voltage of the sixth detection target A6. In view of the above,in the present embodiment, the first period Y1 is shortened when it isdetermined that a leak fault has occurred in the capacitor CA.Consequently, it becomes possible to more accurately acquire the voltageof the sixth detection target A6; thus it also becomes possible to moreaccurately estimate the SOC of the sixth detection target A6 on thebasis of the acquired voltage of the sixth detection target A6.

When a leak fault has occurred in the capacitor CA, in the secondprocess for the sixth detection target A6, the longer the second periodY2, the larger the amount of leak discharge QR from the capacitor CA perunit time and thus the higher the accuracy of estimation of the leakdegree RD of the capacitor CA based on the amount of leak discharge QR.In view of the above, in the present embodiment, the second period Y2 islengthened when it is determined that a leak fault has occurred in thecapacitor CA. Consequently, it becomes possible to more accuratelyestimate the leak degree RD of the capacitor CA; thus it also becomespossible to more suitably determine, based on the estimated leak degreeRD, whether it is necessary to replace the capacitor CA.

Specifically, when the leak degree RD of the capacitor CA is low, it maybe considered that no leak fault has occurred in the capacitor CA, butonly small leak has occurred in the capacitor CA due to, for example,temporary condensation of water vapor. In this case, instead ofreplacing the capacitor CA, a predetermined leak correction may be madewhen acquiring the voltages of the detection targets A1-A6 based on therespective first voltages V1.

Moreover, even when the leak degree RD of the capacitor CA is high, ifthe leak degree RD is stable, it is still possible to make, withoutreplacing the capacitor CA, the predetermined leak correction accordingto the leak degree RD when acquiring the voltages of the detectiontargets A1-A6 based on the respective first voltages V1. Consequently,it becomes possible to use the assembled battery 10 until replacement ofthe capacitor CA due to a leak fault occurring therein.

Second Embodiment

A voltage detection apparatus 20 according to the second embodiment hasa similar configuration to the voltage detection apparatus 20 accordingto the first embodiment. Accordingly, the differences therebetween willbe mainly described hereinafter.

In the voltage detection apparatus 20 according to the first embodiment,the capacitor section 23 includes only the single capacitor CA (see FIG.1).

In contrast, in the voltage detection apparatus 20 according to thepresent embodiment, as shown in FIG. 8, the capacitor section 23includes a serially-connected capacitor pair consisting of a firstcapacitor CA1 and a second capacitor CA2.

Moreover, in the present embodiment, a first terminal N1 is provided asa connection terminal at one end of the serially-connected capacitorpair. A second terminal N2 is provided as a connection terminal betweenthe first capacitor CA1 and the second capacitor CA2. A third terminalN3 is provided as a connection terminal at the other end of theserially-connected capacitor pair.

Each of the electrode terminals Tn of the assembled battery 10 isconnected with one of the first, second and third terminals N1, N2 andN3 via a corresponding one of the switches SWn.

Specifically, to the first terminal N1, there are connected the thirdand fifth electrode terminals T3 and T5 respectively via the third andfifth switches SW3 and SW5. To the second terminal N2, there areconnected the second and sixth electrode terminals T2 and T6respectively via the second and sixth switches SW2 and SW6. To the thirdterminal N3, there are connected the first, fourth and seventh electrodeterminals T1, T4 and T7 respectively via the first, fourth and seventhswitches SW1, SW4 and SW7.

The output-side switch section 24 includes three switches SWA, SWB andSWC that are respectively connected with the first, second and thirdterminals N1, N2 and N3 of the serially-connected capacitor pairconsisting of the first and second capacitors CA1 and CA2.

Moreover, in the present embodiment, the voltage detection unit 25includes a first voltage detection unit 25A and a second voltagedetection unit 25B. The first voltage detection unit 25A is connectedwith the first capacitor CA1 to detect the voltage of the firstcapacitor CAL The second voltage detection unit 25B is connected withthe second capacitor CA2 to detect the voltage of the second capacitorCA2.

The switches SWA and SWB are provided to open and close electrical pathsbetween the first capacitor CA1 and the first voltage detection unit25A. The switches SWB and SWC are provided to open and close electricalpaths between the second capacitor CA2 and the second voltage detectionunit 25B. Specifically, the switch SWA is connected between the firstterminal N1 and the first voltage detection unit 25A. The switch SWB isconnected between the second terminal N2 and both the first and secondvoltage detection units 25A and 25B. The switch SWC is connected betweenthe third terminal N3 and the second voltage detection unit 25B.

The control unit 27 is provided to control the on/off of each of theswitches SWn, the on/off of each of the switches SWA, SWB and SWC, andvoltage acquisition timings TD1 and TD2 of the first and second voltagedetection units 25A and 25B.

Specifically, in the present embodiment, the control unit 27 performs avoltage detection process which includes, as shown in FIG. 9, a firstprocess for acquiring the voltages of the first and second detectiontargets A1 and A2, a first process for acquiring the voltages of thethird and fourth detection targets A3 and A4, and a second process foracquiring the voltages of the fifth and sixth detection targets A5 andA6 and calibrating the first and second voltage detection units 25A and25B.

Specifically, in the voltage detection process according to the presentembodiment, first, the voltages of the first and second detectiontargets A1 and A2 are simultaneously acquired using the first and secondcapacitors CA1 and CA2. Then, the voltages of the third and fourthdetection targets A3 and A4 are simultaneously acquired using the firstand second capacitors CA1 and CA2. Thereafter, the voltages of the fifthand sixth detection targets A5 and A6 are simultaneously acquired usingthe first and second capacitors CA1 and CA2. After the acquisition ofthe voltages of all the detection targets A1-A6, both the first andsecond voltage detection units 25A and 25B are calibrated.

That is, the voltage detection apparatus 20 according to the presentembodiment is a single flying-capacitor type voltage detection apparatuswhich includes the serially-connected capacitor pair consisting of thefirst and second capacitors CA1 and CA2.

In the present embodiment, the first process is performed for both thefirst and second detection targets A1 and A2 during a correspondingdetection period YD. Then, the first process is performed for both thethird and fourth detection targets A3 and A4 during a correspondingdetection period YD. Thereafter, the second process is performed forboth the fifth and sixth detection targets A5 and A6 during acorresponding detection period YD and a subsequent calibration periodYP.

Specifically, in the second process, first and second voltages V1 andV2, which are voltages of the first capacitor CA1 corresponding to thevoltage of the fifth detection target A5, are acquired respectively at afirst-voltage acquisition timing TD1 and a second-voltage acquisitiontiming TD2; then, based on the acquired first and second voltages V1 andV2, it is determined whether a leak fault has occurred in the firstcapacitor CAL Meanwhile, first and second voltages V1 and V2, which arevoltages of the second capacitor CA2 corresponding to the voltage of thesixth detection target A6, are acquired respectively at thefirst-voltage acquisition timing TD1 and the second-voltage acquisitiontiming TD2; then, based on the acquired first and second voltages V1 andV2, it is determined whether a leak fault has occurred in the secondcapacitor CA2.

As described above, the voltage detection apparatus 20 according to thepresent embodiment includes the first and second capacitors CA1 and CA2.The leak fault determination is made for the first capacitor CA1 usingthe first capacitor CA1 itself; at the same time, the leak faultdetermination is also made for the second capacitor CA2 using the secondcapacitor CA2 itself. Consequently, it becomes possible to suitably makethe leak fault determination for each of the capacitors CA1 and CA2regardless of the number of the capacitors included in the voltagedetection apparatus 20.

While the above particular embodiments have been shown and described, itwill be understood by those skilled in the art that variousmodifications, changes and improvements may be made without departingfrom the spirit of the present disclosure.

(1) In the above-described embodiments, the assembled battery 10 has theeight battery modules B electrically connected in series with eachother.

However, the number of the battery modules B included in the assembledbattery 10 is not limited to eight, but may alternatively be greaterthan or equal to two and less than eight, or greater than or equal tonine.

(2) In the above-described embodiments, of the six detection targetsA1-A6, the third and fourth detection targets A3 and A4 each include twobattery modules B while the other detection targets A1, A2, A5 and A6each include only a single battery module B. As an alternative, all thedetection targets A1-A6 may each include only a single battery module B.

(3) In the above-described embodiments, each of the detection lines Lnhas one current-limiting resistor R provided thereon. As an alternative,each of the detection lines Ln may have no current-limiting resistor Rprovided thereon.

(4) In the above-described first embodiment, when it is determined thata leak fault has occurred in the capacitor CA, in each of the first andsecond processes, the first period Y1 is shortened by advancing thefirst-voltage acquisition timing TD1 while keeping the timings ofclosing and opening the switches SWA and SWB unchanged (see FIG. 6).

As an alternative, as shown in FIG. 10, when it is determined that aleak fault has occurred in the capacitor CA, in each of the first andsecond processes, the first waiting period YW1 may be shortened toadvance the timing of closing the switches SWA and SWB. Consequently, itbecomes possible to further advance the first-voltage acquisition timingTD1 from the timing t23 or t33 shown in FIG. 6 to an earlier timing t43or t53 shown in FIG. 10. As a result, it becomes possible to increasethe absolute value of the first voltage V1 acquired at the first-voltageacquisition timing TD1, thereby more suitably suppressing decrease inthe estimation accuracy of the SOC of the corresponding detectiontarget.

(5) In the above-described first embodiment, when it is determined thata leak fault has occurred in the capacitor CA, in the second process,the second period Y2 is lengthened by advancing the first-voltageacquisition timing TD1 while keeping the second-voltage acquisitiontiming TD2 unchanged (see FIG. 6).

As an alternative, the second period Y2 may further be lengthened bydelaying the second-voltage acquisition timing TD2. In this case, thesecond-voltage acquisition timing TD2 may be delayed by delaying thetiming of opening the switches SWA and SWB after the acquisition of thesecond voltage V2.

(6) In the above-described first embodiment, the threshold value Vthused for the leak fault determination of the capacitor CA is presetbased on both the first voltage V1 and the second period Y2. As analternative, the threshold value Vth may be preset based on only thefirst voltage V1 or only the second period Y2. That is, the thresholdvalue Vth may be preset based on at least one of the first voltage V1and the second period Y2.

What is claimed is:
 1. A voltage detection apparatus for an assembledbattery, the assembled battery having a plurality of battery cellselectrically connected in series with each other, the battery cellsbeing divided into a plurality of detection targets, each of thedetection targets including at least one of the battery cells, thevoltage detection apparatus comprising: a capacitor; input-side switchesprovided to parallel connect the detection targets to the capacitor andopen and close electrical paths between the detection targets and thecapacitor; a voltage detection unit configured to detect a voltage ofthe capacitor; output-side switches provided to open and closeelectrical paths between the capacitor and the voltage detection unit; acapacitor charging unit configured to turn the output-side switches toan open state and those of the input-side switches which correspond to aspecific detection target to a closed state and thereby charge thecapacitor with electric power stored in the specific detection target,the specific detection target being one of the plurality of detectiontargets; a first-voltage acquiring unit configured to turn, after thecharging of the capacitor by the capacitor charging unit, the input-sideswitches corresponding to the specific detection target to an open stateand the output-side switches to a closed state and thereby acquire afirst voltage, the first voltage being the voltage of the capacitorafter a first period has elapsed from the turning of the input-sideswitches corresponding to the specific detection target to the openstate; a second-voltage acquiring unit configured to acquire a secondvoltage, the second voltage being the voltage of the capacitor after asecond period has elapsed from the acquisition of the first voltage bythe first-voltage acquiring unit; and a fault determining unitconfigured to determine, based on the first voltage acquired by thefirst-voltage acquiring unit and the second voltage acquired by thesecond-voltage acquiring unit, whether a leak fault has occurred in thecapacitor.
 2. The voltage detection apparatus as set forth in claim 1,further comprising a switch-state control unit configured to turn theoutput-side switches to the open state during the second period from theacquisition of the first voltage by the first-voltage acquiring unit tothe acquisition of the second voltage by the second-voltage acquiringunit.
 3. The voltage detection apparatus as set forth in claim 1,wherein the fault determining unit is configured to determine, when avoltage difference between the first voltage acquired by thefirst-voltage acquiring unit and the second voltage acquired by thesecond-voltage acquiring unit is larger than a preset threshold value,that a leak fault has occurred in the capacitor, the threshold valuebeing preset based on at least one of the first voltage acquired by thefirst-voltage acquiring unit and the second period.
 4. The voltagedetection apparatus as set forth in claim 1, further comprising adetection-target voltage acquiring unit configured to acquire a voltageof the specific detection target based on the first voltage acquired bythe first-voltage acquiring unit.
 5. The voltage detection apparatus asset forth in claim 4, further comprising a first-period setting unitconfigured to set the first period to be shorter when it is determinedby the fault determining unit that a leak fault has occurred in thecapacitor than when it is determined that no leak fault has occurred inthe capacitor.
 6. The voltage detection apparatus as set forth in claim1, further comprising a second-period setting unit configured to set thesecond period to be longer when it is determined by the faultdetermining unit that a leak fault has occurred in the capacitor thanwhen it is determined that no leak fault has occurred in the capacitor.